Australia has always been a remote place, and getting there today is no easy task. Fifty thousand years ago, the trek was even trickier: The first human visitors had to navigate 60 kilometers of open ocean just to reach the continent. The journey appears to have been so difficult, in fact, that--according to new research--the ancestors of Australia's first settlers had no contact with the outside world for tens of thousands of years.

Paleontologists have disagreed over how many times Australia was visited by humans over the past 50,000 years. Some have postulated several migrations to explain why some human fossils found in Australia differ enough that they seem to have come from different lineages. For example, the oldest ever found in Australia were of relatively gracile (modern human-like) people. But other remains, 20,000 years more recent, reveal heavy-set individuals with more primitive features. Recent evidence, however, suggests that a single migration took place, with humans whose ancestors originated in Africa making their way from Southeast Asia to New Guinea or northern Australia. The new study, published online this week in the Proceedings of the National Academy of Sciences, adds further credence to this theory.

A team headed by Georgi Hudjashov of Tartu University in Estonia analyzed variation in the Y chromosome and mitochondrial DNA (mtDNA, which is inherited only from women) in several hundred people from Australia and New Guinea. By knowing the mutation rate of mtDNA and comparing Australian samples with those from Asian populations, the team ascertained that the Australian and New Guinean populations branched off from a parent population 50,000 years ago, and that no significant additions to the aboriginal Australian gene pool had been made until modern times. "Australia was colonized, then nobody else came," says co-author Peter Underhill of Stanford University in Palo Alto, California.

This further bolsters the "out of Africa" theory--that all modern humans' ancestors originated in Africa and did not, as some have argued, arise from regional populations that evolved from earlier hominids, the researchers say. What's more, the authors say, the genetic data show that that Australians diverged from New Guineans about 20,000 years ago, indicating that the populations became separate long before the land bridge between them disappeared under the waves 8000 years ago.

The findings confirm the "quite extraordinary" picture of Australians and Melanesians living in isolation from the rest of the world for tens of thousands of years, says geneticist Jonathan Friedlaender of Temple University in Philadelphia, Pennsylvania. The pattern of separation of land masses is so old that most mammals never made it to Australia or New Guinea until a few thousand years ago, says Friedlaender.